The present invention relates to microelectronic packaging and more particularly relates to methods of making connectors and packaged microelectronic components. In various microelectronic devices, it is desirable to provide a connection between two components, which can accommodate relative movement between the components. For example, where a semiconductor chip is mounted to a circuit board, thermal expansion and contraction of the chip and circuit board can cause the contacts on the chip to move relative to the corresponding electrically conductive features of the circuit board. This can occur during service and can also occur during manufacturing operations as, for example, during soldering operations on the circuit board.
As illustrated in certain preferred embodiments of U.S. Pat. No. 5,518,964 (xe2x80x9cthe ""964 patentxe2x80x9d) movable interconnections between elements such as a semiconductor chip and another element can be provided by first connecting leads between the elements and then moving the elements away from one another so as to bend the leads. For example, a connection component may incorporate a dielectric body and leads extending along a bottom surface of the dielectric body. The leads may have first or fixed ends permanently attached to the dielectric element and connected to electrically conductive features such as terminals, traces or the like on the dielectric body. The leads may also have second ends releasably attached to the dielectric body. The dielectric body, with the leads thereon, may be juxtaposed with the chip and the second ends of the leads may be bonded to contacts on the chip. Following bonding, the dielectric body and chip are moved away from one another, thereby bending the leads. During or after movement, a curable material such as a liquid composition is introduced between the elements. This is cured to form a compliant dielectric layer such as an elastomer or gel surrounding the leads. The resulting packaged semiconductor chip has terminals on the dielectric body connection component which are electrically connected to the contacts on the chip but which can move relative to the chip so as to compensate for thermal effects. For example, the packaged chip may be mounted to a circuit board by solder-bonding the terminals to conductive features on the circuit board. Relative movement between the circuit board and the chip due to thermal effects is taken up in the moveable interconnection provided by the leads and the compliant layer.
Numerous variations of these processes and structures are disclosed in the ""964 patent. For example, the package-forming process can be conducted on a wafer scale, so that the numerous semiconductor chips in a unitary wafer are connected to connection components in one sequence of operations. The resulting packaged wafer is then severed so as to provide individual units, each including one or more of the chips and portions of the dielectric body associated therewith. Also, the leads may be formed on the chip or wafer rather than on the dielectric body. In further embodiments, also disclosed in the ""964 patent, a connector for use in making connections between two other microelectronic elements is fabricated by a generally similar process. For example, in one embodiment a dielectric body having terminals and leads as discussed above is connected to terminal structures on a temporary sheet. The temporary sheet and dielectric body are moved away from one another so as to bend the leads, and a liquid material is introduced around the leads and cured so as to form a compliant layer between the temporary sheet and the dielectric body. The temporary sheet is then removed, leaving the tip ends of the terminal structures projecting from a surface of the compliant layer. Such a component may be used, for example, by engaging it between two other components. For example, the terminal structures may be engaged with a semiconductor chip, whereas the terminals on the dielectric body may be engaged with a circuit panel or other microelectronic component. Thus, the broad invention taught in the ""964 patent offers numerous desirable ways of making electrical interconnections and connectors.
Additional variations and improvements of the processes taught in the ""964 patent are disclosed in commonly assigned U.S. Pat. Nos. 5,578,286; 5,830,782; and 5,688,716 and in copending, commonly assigned U.S. patent application Ser. No. 08/690,532, filed Jul. 31, 1996 and Ser. No. 09/271,688, filed Mar. 18, 1999, the disclosures of which are hereby incorporated by reference herein.
The present application is directed to specific embodiments of the ""964 patent process and certain embodiments thereof.
One aspect of the invention provides methods of making a microelectronic assemblies. The methods in accordance with this aspect of the invention desirably include a the steps of providing leads physically connected to a bottom surface of a support, each said lead having a tip end and a terminal end and engaging the support with a microelectronic element as, for example, a chip, a wafer or an assemblage of plural discrete chips, having contacts thereon so that the tip ends of the leads are aligned with the contacts of the microelectronic element. The methods further include bonding the tip ends of the leads to the contacts; and then after such bonding, selectively degrading the connection between the support and the leads at and adjacent the tip ends thereof so as to free the tip ends from the support and leave the terminal ends secured to the support. Preferably, the methods include the further step of moving the support and microelectronic element through a predetermined displacement away from one another after degrading the connection between the tip ends and the support so as to deform said leads towards a vertically-extensive disposition. Optionally, the connection between the terminal ends of the leads and the support may be degraded after the moving step, so as to free the support and allow removal of the support. A flowable material may be introduced around the leads during or after the movement step to form a dielectric layer surrounding said leads.
Where the material connecting the tip ends to the support is radiation-sensitive, the step of selectively degrading the connection may include selectively applying radiation through said support at and adjacent to the tip ends of the leads. Thus, in methods according to this aspect of the invention, there is no need to fabricate precise mechanical features such as frangible connections to hold the leads in place until they can be bonded to the microelectronic element. Instead, the tip ends of the leads are constrained reliably until such constraint is released by selective degradation of the connecting layer.
A related aspect of the invention provides methods of connecting a plurality of leads to one or more microelectronic elements. Methods according to this aspect of the invention desirably include the steps of providing the leads physically connected to a support by a connecting material so that said leads are maintained in position on the support at least partially by the connecting material; juxtaposing the support with the microelectronic element so that the leads are aligned with contacts on the microelectronic element and bonding the leads to the contacts of the microelectronic element. After the bonding step, the connection between the leads and the support is released by degrading the connecting material. The step of degrading the connecting material may include directing radiant energy through the support onto said connecting material. Preferably, the leads are flexible after they are released from the support. As further explained below, certain methods according to this aspect of the invention provide for conversion of constrained, inflexible leads to a flexible state simply by releasing the leads from the support, with or without a further step such as bending the leads.
A further aspect of the invention provides methods of making a packaged microelectronic component. Methods according to this aspect of the invention desirably include the steps of providing a support including a structural layer transparent to radiation in a degradation wavelength band and electrically conductive elements secured to said structural layer by a connecting layer on a bottom surface of said structural layer. The conductive elements are connected to a microelectronic component, and then the conductive elements are released from the structural layer by directing radiation in said degradation wavelength band through said structural layer to degrade the connecting layer. The conductive elements or features provided on the support may include leads as discussed above; individual conductive terminals; or conductive terminals incorporated in subassemblies also including dielectric components. The step of connecting the conductive features carried by the support to a microelectronic component may include providing leads extending between said conductive features and the microelectronic component.
The conductive features may be carried on a sacrificial layer having etching properties different from those of the conductive features such that the sacrificial layer can be etched without destroying the conductive features the sacrificial layer being connected to said structural layer by said connecting layer. To provide such different etching properties, the sacrificial layer may be formed from a material different from the material constituting said conductive features. Alternatively or additionally, the sacrificial layer may be formed from the same material as the conductive features but in a thickness substantially less than the thickness of the conductive features. Thus, degradation of the connecting layer frees the sacrificial layer from the structural layer. The method may further include etching the sacrificial layer to remove it without destroying the conductive features. The sacrificial layer can be used to convey plating or etching currents during formation of the conductive features.
A related aspect of the invention provides a support or mandrel for forming microelectronic elements incorporating a structural layer transparent to radiation in a degradation wavelength band; an electrically conductive sacrificial layer thinner than the structural layer; and a connecting layer securing said sacrificial layer to said structural layer, said connecting layer degradable by radiation in said degradation wavelength band.
Yet another aspect of the invention provides a structure for forming microelectronic assemblies. The structure includes a rigid support having a substantially uniform coefficient of thermal expansion and a plurality of electrically conductive elements connected to said support by a connecting material, said support being transparent to radiation in a band of wavelengths effective to degrade said connecting material. Such a structure can be used, for example, in the methods discussed above. The electrically conductive elements on such structure may include features such as leads and terminals. The element may further include one or more sheetlike dielectric layers, the terminals being exposed at a top face of said dielectric layer facing toward said support.
A still further aspect of the invention provides a method of making a plurality of packaged microelectronic components. The method according to this aspect of the invention includes the steps of providing (i) a temporary support with a plurality of separate dielectric elements thereon, each such dielectric element having electrically conductive features thereon; (ii) a microelectronic unit including a plurality of microelectronic devices, and (iii) a plurality of leads, the leads having first ends connected to conductive features on the dielectric elements and having second ends attached to said microelectronic devices. Once these elements have been provided, the temporary support is at least partially removed so as to separate the dielectric elements from one another. Methods according to this aspect of the invention include the realization that, when a unitary dielectric sheet is connected to a relatively large microelectronic unit such as a unitary semiconductor wafer, the support may constrain the thermal expansion of the sheet so as to suppress differential expansion and contraction during to attachment process. However, when the support is removed, the sheet tends to spring back to its unconstrained size. This tendency is restrained by the wafer, leads and encapsulant. However, this tendency may impose internal stress in the assembly, which may damage or distort the assembly. However, when smaller, individual dielectric sheets, also referred to herein as xe2x80x9ctilesxe2x80x9d are employed, the internal stresses can be reduced substantially, typically by one or more orders of magnitude. Moreover, because these tiles are present on the support during the steps used to connect the conductive features to the microelectronic device, the support maintains the conductive features in the correct spatial relationship for alignment with the contacts or other conductive features on the microelectronic element.
The connection between the tiles and a microelectronic element such as a wafer may be made by means of leads carried on the bottom surfaces of the tiles or on the top surface of the wafer. Most preferably, the step of providing the temporary support with said dielectric elements includes fabricating said dielectric elements and conductive elements on the temporary support. Provided that the support has a predictable coefficient of thermal expansion, the conductive features can be fabricated in precisely-controlled positions. In this aspect of the invention as well, the temporary support may include features such as a radiation-transmissive structural layer and radiation-degradable connecting layer to permit release of the tiles from the support. The support may also include an etchable sacrificial layer.
A related aspect of the invention provides a component for making packaged microelectronic elements. The component includes a support having a structural layer with a substantially uniform, isotropic coefficient of thermal expansion, and a plurality of separate dielectric elements releasably attached to said support structure, said dielectric elements having conductive features thereon. Desirably, the support is formed from a material transparent to radiation of a predetermined degradation wavelength, and the dielectric elements are secured to the structural layer by a connecting material degradable by radiation in such degradation wavelength band. Merely by way of example, the degradation wavelength band may be in the ultraviolet range, the visible range, or the infrared range, although other wavelengths may be used. The transparent material desirably has a coefficient of thermal expansion of about 6xc3x9710xe2x88x926/xc2x0C. or less, so that the transparent material is CTE-matched to silicon to within a reasonable tolerance
A further aspect of the invention provides methods of making microelectronic assemblies. Methods according to this aspect of the invention desirably include the steps of providing a semiconductor element such as a wafer including one or more semiconductor chips, said semiconductor element having contacts on a front surface and forming leads in place on the semiconductor element overlying the front surface, said leads having contact ends connected to the contacts and having tip ends releasably connected to the semiconductor element; then juxtaposing said semiconductor element and leads with a further element such as a support and/or dielectric element having pads thereon, and bonding said tip ends of said leads to said pads. Most preferably, the pads are larger than the contacts of the chip and desirably wider than the ends of the leads connected to the pads. As further discussed below, this aspect of the present invention incorporates the realization that where the leads on the chips are aligned to pads wider than the ends of the leads, the process can operate satisfactorily even with a relatively large alignment tolerance. Typically, the contacts on the chip are disposed at first center-to-center distances from one another and the pads are disposed at second center-to-center distances larger than said first center-to-center distances. As also discussed below, this provides room for the pads to have relatively large diameter.
A related aspect of the invention provides an element for forming microelectronic assemblies. The element desirably includes a rigid support having a substantially uniform coefficient of thermal expansion and a plurality of electrically conductive structures defining pads facing away from said support, the conductive structures being releasably connected to said support, the pads desirably being about 150 xcexcm to about 400 xcexcm in diameter.
These and other objects, features and advantages of the invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.